Electrical filters are utilized to transmit desired electrical signals by selecting or rejecting one or more signal components, related to frequency. Filters may generally be grouped as lowpass, highpass, bandpass and bandstop filters depending on their characteristics. A lowpass filter passes low frequency electrical signals, while rejects high frequency electrical signals. Conversely, a highpass filter suppresses the frequency electrical signals, while passes high frequency signals. A bandstop filter, however, passes all signals except the signals having frequencies between two selected frequency points. A bandpass filter, however, passes electrical signals in a particular frequency band between two frequency points (e.g., a high and low frequency point).
Depending on the implementation of the filter, the types of filters can also be grouped into three types, i.e., lumped, distributed and semi-lumped (which is constructed by lumped and distributed elements). The size of distributed elements, which is usually a section of the transmission line, is determined by the wavelength associated to the operational frequencies, such that lower frequencies translate into larger distributed elements. Accordingly, at frequencies lower than 200 Mhz, semi-lumped type or distributed type filters are impractical due to the unacceptably large size of the distributed elements. However, in the higher microwave frequency and millimeter-wave frequency regions, distributed type filters are commonly used due to their acceptable size and typically better performance than the lumped type filters. Lumped type filters are conventionally not suitable at frequencies higher than several hundred MHz as they become too lossy and are susceptible to high parasitic effects. Although the problem of signal loss in lumped elements can be improved in superconductor structures, such applications are limited.
At frequencies between several hundred MHz to several GHz, the relatively large size of distributed elements is not practical, e.g., in mobile communication instruments. The trends in designing mobile communication instruments are to reduce both physical size and power consumption. There are two typical ways to design a high performance miniaturized filter. The first is utilizing the semi-lumped configuration, mentioned above, and the second is utilizing a high dielectric constant structure. Semi-lumped type filters usually employ chip capacitors, interdigital type capacitors and/or metal-insulation-metal (MIM) capacitors, which are not as lossy as lumped inductors, and sections of distributed transmission lines which are usually much shorter than 1/4 signal wavelength. Besides the miniaturization advantages of the semi-lumped configuration, as compared to distributed filters, this type of filter also has the ability to control the suppression of the periodic spurious signals that distributed type filters usually suffer.
Recently, high dielectric constant ceramic filters, such as coaxial or mono-block types, have become very popular due to their high performance and small size. The high performance is due, in part, to the round or smooth curves exhibited in the cross section of the transmission lines generate less conductor loss. The small size is due, in part, to the fact that the dielectric constant of ceramic is very high, resulting in a reduction of the signal wavelength. Since these kind of bandpass filters usually use 1/4 wavelength transmissions as resonators or "J, K" inverters, they are not suitable to be implemented in low dielectric constant structures having a frequency region between several hundred MHz to several Ghz.
In addition, there are several other disadvantages utilizing ceramic filters. Due to their 3D profiles, each design requires a new molding model which is usually very expensive. Further, in fabrication of the coaxial type ceramic filter, different coaxial resonators are separately sintered and coated, and the blocks are electrically individually connected to each other, usually by soldering the connecting wires by hand. Further, the separate blocks must be fastened to some mounting support in a mechanically reliable way. The above indicates that the manufacture is difficult and costly. Although mono-block type ceramic filters are improvements of the coaxial type ceramic filters, they have essentially reached their limits with respect to miniaturization.
In addition, the low temperature cofirable ceramic (LTCC) technique has become popular in RF applications. Multi-layer ceramic (MLC) integrated circuits based on the LTCC technique have several advantages, such as the capability of mass production, 3D high integration, as well as comprising buried resistors, inductors and capacitors. Further, almost all the passive components can be fabricated in one manufacture processing. Except that a portion of the MLC circuit surface area is used for mounting active devices, almost all passive components can be buried into different ceramic layers to save real estate. Accordingly, the size of RF module can be reduced.
However, as stated with respect to MLC circuits modules, the use of ceramic type occupies some the surface area and requires additional processing steps to mount the active devices on the surface. Such use requires a larger surface area and increases manufacture complexity. Accordingly, there is a need for filters suitable to be fabricated and implemented in MLC structures that does not inhibit circuit miniaturization.
Recently, laminated planar bandpass filters have been developed for MLC structures. These filters consist of planar resonators that are buried in a high dielectric constant MLC structure, and can therefore be grouped as semi-lumped type filters. These bandpass filters use coupled transmission line sections and lumped planar capacitor pads for the coupling capacitor of the transmission line resonators. Such laminated planar filters may be treated as the variations of comb-line or stepped digital elliptic filters. However, the use of coupled transmission lines is disadvanteous because in MLC structures having low dielectric constants, the signal wavelength is relatively large. Utilizing coupled transmission lines further increases miniaturization concerns. In addition, the flexibility in designing laminated planar bandpass filters is inhibited by the control criteria of coupled transmission lines.
Accordingly, it is an object of the present invention to overcome the deficiencies in the prior art.